Hmm, this sounds good but it makes me wonder: Would there be a lucrative market for scammers? Everything else is details, as you say, but that is one of the more important details. Aside from that, bun.

"Would there be a lucrative market for
scammers?"
Since the winning bidder pays the charity
directly (and the seller gets nothing) there
is no profit motive for selling fake items.
There is still a potential for scammers to
lie saying that they haven't received items,
though.

I skipped on some of the details in the
idea because I wanted to keep it simple
and save the details for the annotations.

There are a few alternatives that spring
to mind.
1. The buyer pays for shipping --
possibly asking the seller (who gets
nothing after all) to be a little too
charitable. Easy to administrate and
hard to scam, though.
2. The charity pays -- Either a rebate
into the sellers auction account or by
providing the seller with pre-pay labels.
Either way has some overhead for the
charity.
3. The charity ships -- I can't figure out
why I like this so much (I do, though),
particularly as you end up taking the
item to the charity shop anyway.